The proposed project is designed to gain insight to the interactions and associations between Borrelia pathogens and their respective zoonotic vertebrate reservoirs and arthropod vectors. To fully characterize the structure of natural populations of B. burgdorferi, it is necessary to sample isolates from the various levels of its organization to determine the temporal, geographic, and host environmental factors which determine the distribution and exchangeability among B. burgdorferi isolates. Thes geographic requirements of this sampling are met by the use of coastal islands, which have been chosen for the unique qualities that make them ideal natural laboratories for tracking the ecological dynamics of emerging disease. The temporal sampling of these two sites is accomplished both longitudinally (by sampling each islands bi- annually for each year of the study), and by cross-sectional sampling of the tick cohorts in non-overlapping annual cycle. The relationship between the host environments can be tested directly, since the Borrelia isolates are collected directly from their hostsj. The multi-locus genotypes of all mice, ticks, and Borrelia isolates from each island site (and at each time point) will be determined, thus providing a rich resource for evaluating the structure of their respective populations, and the interactions among these species.